Design B
Disclaimer: This is not about culture-specific penile size or sexual expertise or ineptitude. Whatever that reality, this slogan comes from the American spate of sexual braggadocio back in the 70s started by "Truckers make better lovers". ("Electricians do it with less resistance" et alia.)"Gaijin make better aijin" works because "gaijin" rhymes with "aijin". So it works even if you don't know the roots. It works even better if you do.